1. Field of the Invention
The present invention relates to an interference rejection device that rejects radar interference occurring between a plurality of radar devices, and to a radar device equipped with an interference rejection device, and to an interference rejection method for rejecting radar interference.
2. Background Information
A conventional marine radar transmits a sine wave pulse by using a magnetron. The amplitude of the interference signal produced when this radar (hereinafter referred to as a magnetron radar) is affected by another magnetron radar (such as a radar installed on a nearby ship) is far greater than the amplitude of a signal pertaining to a wave reflected from a target (hereinafter referred to as an echo signal). With the conventional magnetron radar, the effect of this interference signal is suppressed based on received signals from a plurality of continuous sweeps. For example, the amplitudes of the received signals at a given distance and obtained in three continuous sweeps are compared, the minimum value is selected, and this minimum value is used as the amplitude of a target sweep out of the three times.
In recent years, however, a pulse compression radar, in which a semiconductor amplifier is used instead of a magnetron to transmit a modulated pulse, has begun to see practical use as marine radar. With the pulse compression radar, a pulse with a longer time width T than that of magnetron radar is transmitted, and the received signal is put through a matched filter (hereinafter abbreviated as MF) corresponding to this transmitted pulse, which compresses the pulse width and also increases the signal-to-noise ratio (S/N). When an interference signal from magnetron radar undergoes this processing, it is expanded to about the same as the time width T of the interference signal. Furthermore, if an interference signal from a pulse compression radar of the same type as that installed on another ship undergoes MF processing, the time width of the interference signal will expand to about 2T. Therefore, with the pulse compression radar, it is preferable to reject any interference signals prior to MF processing.
However, if the interference rejection processing discussed above is applied to the pulse compression radar, not only the interference signal, but also the echo signal will end up being changed, so the following distortion occurs in the MF output.
a. The peak level is reduced.
b. The pulse width is increased (resolution is degraded).
c. There is an increase in range side lobes.
To solve these problems, various devices and methods have been proposed in which an interference signal is detected from a signal prior to MF processing in a pulse compression radar, and just the interference signal is selectively rejected, as discussed, for example, in Patent Literature 1 (Japanese Laid-Open Patent Application No. 2008-96337).
The interference rejection method discussed in Patent Literature 1 basically detects an interference signal by going through the following three procedures.
Procedure 1: The amplitude of target data, and the amplitude of reception data at a given distance in a plurality of sweeps before and after the target data are sorted by magnitude of the amplitude value.
Procedure 2: The amplitude value of a specific number reception data is multiplied by a constant to obtain a threshold for use in determining interference.
Procedure 3: The threshold is compared to the amplitude of the target data, and if the target data is greater, it is concluded that there is a disturbing wave (interference signal) in the target data.